1. Field of the Invention
The present invention relates to a wireless transmitter and related multimedia system, and more particularly, to a wireless transmitter capable of transmitting a frequency modulation signal with a DC source signal and related multimedia system.
2. Description of the Prior Art
Satellite radio is a broadcasting technique following amplitude modulation (AM) and frequency modulation (FM) radio, which has better spectral utilization efficiency for solving a problem of the lack of available broadcasting cannels. Moreover, vehicle communication and multimedia service, such as satellite navigation, radio and video programs, etc, are gradually taken account of for the public. However, a general vehicle multimedia system can only receive AM and FM radio, but has no built-in function of receiving satellite radio. If a user wants to listen to a satellite radio program during driving, a satellite radio receiver needs to be installed in the vehicle for high-quality satellite radio and additional data service functionality.
The conventional satellite radio receiver is not only used for demodulating satellite radio signals, but is also used for transmitting content of satellite radio to a frequency modulation (FM) demodulator of the vehicle in an FM signal form. After the content of the satellite radio is transmitted to the FM demodulator, the content of the satellite radio is played by audio equipment. In addition, a cigarette lighter adapter (CLA) is an integral part of vehicle electronic equipment, and is used for adapting power to a mobile phone, a multimedia player or a satellite radio receiver, etc. The conventional CLA has a built-in FM signal transmission function for transmitting music content stored in the external multimedia player to the FM demodulator to generate a demodulated audio signal, and then the demodulated audio signal is played by the audio equipment.
Please refer to FIG. 1, which is a schematic diagram of a vehicle multimedia system 10 according to the prior art. The vehicle multimedia system 10 includes a satellite radio receiver 110, a CLA 120, a cable 130, an FM receiving antenna 140, an FM demodulator 150, and a speaker 160. The satellite radio receiver 110 includes a satellite receiving antenna 112, a signal converting circuit 114, and an FM transmitting antenna 116. The signal converting circuit 114 is used for converting a satellite radio signal received from the satellite receiving antenna 112 into an FM signal, and transmitting the FM signal to the air through the FM transmitting antenna 116. The CLA 120 is coupled to a power supply 12, and is used for converting power outputted from the power supply 12 into power required by the satellite radio receiver 110, and transmitting the power to the satellite radio receiver 110 though the cable 130. The CLA 120 includes an FM transmission circuit 122 and an FM transmitting antenna 124. The FM transmitting circuit 122 is used for converting a desired audio signal into an FM signal, and transmitting the FM signal through the FM transmitting antenna 124. The FM receiving antenna 140 receives the FM signal transmitted from the FM transmitting antenna 116 of the satellite radio receiver 110 or the FM transmitting antenna 124 of the CLA 120, and then transmits the FM signal to the FM demodulator 150 for demodulation. Finally, the demodulated audio signal is transmitted to the speaker 160 and is played.
For the conventional vehicle multimedia system 10, the FM transmitting antenna 116 of the satellite radio receiver 110 is usually installed on vehicle housing, and has to be closed enough to the FM receiving antenna 140, which causes uneasy installation. The FM transmitting antenna 124 of the CLA 120 is usually installed on an internal printed circuit board of the CLA 120, and is rarely an exposed antenna. Therefore, the FM transmitting antenna 124 is easily restricted by a shape of the structure and has poor FM signal transmission function, which decreases the quality of the audio signal. At present, the FM transmission function of the satellite radio receiver 110 and the CLA 120 have not reached the optimization state.